The Big Bang Theory is a broadly accepted theory which give information of the origin and evolution of the universe. According to this theory the universe was only a few millimeters across before 12 to 14 billion years ago. On that time it was very dense and hot. But due to big bang it converts in vast and much cooler that we see in the present time. We can see this factor by microwave detection.

 

expanding-universe-after-big-bang

 

Actually the big bang theory have two main pillars-

  1. General Relativity
  2. The Cosmological Principle

1. General Relativity

Einstein was give a new theory of gravity in 1916 that tells about space-time. Newton’s theory of gravity was valid for rest objects or slow moving objects than the speed of light but Einstein’s theory of gravity was valid for rest and moving objects. According this theory gravity is no longer described by gravitational field but rather it is distortion of space time itself. And this theory was able to account the peculiarities in the orbit of Mercury and the bending of light  by the sun. Both are unexplained by Newton’s theory of gravity.

 

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2. The Cosmological Principle

It tells about how matter distributed in the universe. According to this principle the distribution of matter is homogenius and isotropic. It is distributed equally in all direction at large scale. If wee see the universe at roughly vision then we see that all matters are equally distributed in all direction in the universe.

 

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Foundation of Big-Bang Theory

We know that the matter in the universe is homogenius and isotropic (According to the cosmological principle) and the gravity on matter also affected by Space-Time (According to the general relativity). So it can be shown that the corresponding distortion of space-time can only have one of the three forms as shown in the picture.

 

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  1. It can positively curved like the surface of a ball and finite in extent.
  2. It can be negatively curved like a saddle and infinite in extent.
  3. It can be flat and infinite in extent.

In finite universe if you start a journey on one point and continuing the journey then you reach at the starting point but in infinite universe if you start your journey at one point then you would not reach at that point. Matter plays most important role in cosmology because the average density of matter determines the geometry of the universe. If the density of matter is less than critical density then he universe is open and infinite. If the density of matter is greater than critical density then the universe is closed and finite. If the density of matter is equally to critical density then the universe is flat and infinite. The value of critical density is very small. it is equally to 6 hydrogen atoms per cubic meter. One of the key scientific question in cosmology is that what is average density of matter in the universe? That is not yet known but according to many scientists it appears to close to the critical density.

Universe has a finite age (13.77 billion years). So we can see only finite distance out into space across 13.77 billion light years that called our horizon. It is possible that the universe have more complicated global topology than we can think.

So Big Bang Theory tells about the evolution of the universe. According to Cosmic Microwave Background (CMB) we scientists calculated the time and event after the big bang:

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